1. Field of the Invention
The present invention relates to a variable resistor for a valve opening control sensor of an EGR device of an automobile and an EGR sensor for controlling an air-gasoline mixing ratio, and an encoder to be used as various types of sensors.
2. Description of the Prior Art
A prior art sensor of this type will be explained with reference to a sectional view of FIG. 10. The prior art sensor comprises a casing 1 which forms an outer shell; an operating shaft 2 which is movable from side to side in the drawing with respect to the casing 1; an insulating substrate 3 built in the casing 1; a slider piece 4 which slides in contact with an unillustrated conductive pattern of a resistor and a current collector mounted on the upper surface of the insulating substrate 3; a slider receiver 5 for storing the slider piece 4; a return spring 6 disposed offset from the axis of the operating shaft 2 for applying a spring force to the slider receiver 5; and an external terminal 7 connected to the resistor substrate 3.
In a bearing 1a mounted in one end of the casing 1 the operating shaft 2 is inserted and supported. At the opening 1b provided in the other end a cover 8 is mounted to close the opening. In the opposite inner wall of the casing 1 a pair of guide grooves (not shown) are formed extending in a direction parallel with the axis of the operating shaft 2. A pair of protrusions 5a formed on the slider receiver 5 is engaged with the guide grooves to thereby guide the movement of the slider receiver 5.
The insulating substrate 3 provided within the casing 1 is engaged at both ends in a pair of grooves (not shown) formed in the opposite inner wall of the casing 1. The insulating substrate 3 is pressed to the slider receiver 5 side by a wavy plate spring 9 disposed between the casing 1 and the back side of the insulating substrate 3, thereby being attached in the pair of grooves.
On one end side of the insulating substrate 3 is formed a terminal portion 3a which has an unillustrated terminal pattern of a conductor connected to a resistor and a current collector. Also on one end side of the external terminal 7 a connecting portion 7a is formed. The terminal portion 3a of the insulating substrate 3 and the connecting portion 7a of the external terminal 7 are both disposed on the opening 1b side and are mutually connected at a clip terminal 10. In the casing 1 is formed a groove 11 in which the return spring 6 is mounted offset from the axis of the operating shaft 2, so that the return spring 6 disposed in the groove 11 will elastically press the slider receiver 5 in the axial direction.
Also a protrusion 2a is formed on the operating shaft 2 positioned within the casing 1; with the protrusion 2a held in contact with the bottom surface of a recess 5b provided in the slider receiver 5, the slider receiver 5 elastically presses the operating shaft 2 by the spring force of the return spring 6 towards the arrow G side.
The return spring 6 elastically presses the slider receiver 5 in a position offset from the axial center of the operating shaft 2, and therefore the slider receiver 5 is constantly exerted with a force in the direction of the arrow D shown in FIG. 10 to thereby rotate on a portion which the protrusion 2a of the operating shaft 2 contacts as a fulcrum.
The pair of protrusions 5a formed on the slider receiver 5 have a tapered surface on the illustrated lower surface side; and the pair of guide grooves provided in the case 1 for guiding the protrusion 5a are also so formed as to be in contact with the tapered surface. Therefore, since the slider receiver 5 is hard to move if added with vibration, the pair of guide grooves can come into contact with the tapered surface with stability.
Furthermore, a mounting hole 1d is formed in the mounting portion 1c provided outside of the casing 1, for thereby mounting the sensor on another device.
The prior art sensor of the above-described constitution is mounted in a control valve housing 12 in an unillustrated EGR device for reducing nitrogen oxides included in exhaust gases from the automotive engine.
Within the control valve casing 12, as shown in FIG. 11, a valve rod 15 which moves in a direction parallel with the axis of the operating shaft 2 when the solenoid 14 is energized; on one end of the valve rod 15 a control valve 16 is mounted. The control valve 16 is mounted in the casing 12 in such a manner that it can open and close the exhaust gas return port 17.
The control valve 16 is mounted as a valve opening sensor which senses the amount of opening of the exhaust gas return port 17, by a mounting member 13 in the control valve casing 12.
When the valve rod 15 of the control valve 16 is moved by control of the solenoid 14 in the direction of the arrow F in which the exhaust gas return port 17 is opened, the valve rod 15 pushes the operating shaft 2 which protrudes out of the casing 1, to thereby move the operating shaft 2 towards the inside of the casing 1.
At this time, the protrusion 2a of the operating shaft 2 which is in contact with the bottom surface of the recess 5b of the slider receiver 5 in the casing 1 shown in FIG. 10 pushes the slider receiver 5 to move the slider receiver 5 and the operating shaft 2 in the direction of the arrow F in the casing 1 against the spring force of the return spring 6.
Then, the slider piece 4 sliding in contact with the insulating substrate 3 also moves with the movement of the slider receiver 5, thereby changing the resistance value of the sensor.
The sensor is of such a constitution that as the valve rod 15 moves in the direction of the arrow G in which the exhaust gas return port 17 is closed by the control valve 16, the operating shaft 2 and the slider receiver 4 are also moved towards the arrow G side with the spring force of the return spring 6.
If, however, the current being supplied to the solenoid 14 is cut off because of certain circumstances, the solenoid 14 will fail to control the movement of the valve rod 15, and accordingly the valve rod 15 will become freely movable in the directions of the arrows F and G.
When external vibration is applied to the EGR device under the above-described condition, the spring force of the return spring 6 elastically exerted to the operating shaft 2 which is in contact with the valve rod 15 can not restrict movement of the valve rod; in this state, the valve rod 15 is freely movable in the directions of the arrows F and G. Thus the slider receiver 5 also freely moves with the movement of the valve rod 15, accelerating wear of the slider piece 4 and accordingly resulting in shortened sensor life.
To solve the above-described problem of the prior art sensor, the movement of the valve rod 15 must be restricted even when the current supply to the solenoid 14 is cut off.
In the prior art sensor, therefore, a conceivable means for solving the above-described problem is to increase the spring force of the return spring 6 to provide a greater force for operating the operating shaft 2. Simply increasing the spring force of the return spring 6, however, increases a force to turn the slider receiver 5 to the direction of the arrow D; accordingly the slider receiver 5 will fail to smoothly move within the casing 1, resulting in poor controllability.